1. Field of the Invention
The present invention relates to high voltage dropout type fuse cutout devices and more particularly to a full service dropout fuse comprising a current limiting fuse section operative only during the interruption of heavy fault currents and an expulsion fuse section serially connected to the current limiting fuse section for interrupting lower fault currents such as transformer secondary fault currents.
2. Description of the Prior Art
High voltage dropout fuse cutouts are known to the art as illustrated by U.S. Pat. No. 3,041,426 -- Baker issued June 26, 1962 and assigned to the same assignee as the present application. Such dropout type cutouts conventionally comprise a fuse body mounted between two in-line terminals by a toggle linkage arrangement so that upon fuse operation, the toggle linkage operates to permit the fuse body to disengage the upper terminal and pivot free of the upper terminal.
In addition, the serial combination of current limiting fuses with expulsion type fuses to increase the spectrum of current interrupitng capabilities of the fuse device is also known in the art. For example, U.S. Pat. No. 2,917,605 -- Fahnoe discloses the serial combination of a current limiting fuse section and an expulsion fuse section. Similarly, U.S. Pat. No. 3,827,010 -- Cameron et al. discloses the utilization of a serial combination of a current limiting fuse section and an expulsion fuse section in a conventional dropout type cutout fuse.
However, neither the Fahnoe nor the Cameron et al. patent recognizes that there is a unique voltage distribution that occurs within such a serial combination of a current limiting fuse section and an expulsion fuse section. Thus, even though the overall length of the fuse device may appear to be sufficient to provide adequate external dielectric strength even under contaminated or wet conditions, it has been discovered that additional measures must be taken to provide sufficient external dielectric strength of such devices under contaminated or wet conditions.